This invention relates to an incandescent lamp that is turned on with a coil axis of a filament being directed in a vertical direction, and, for example, an incandescent lamp filled with an inert gas containing halogen.
In recent years, the range of utilization of incandescent lamps acting as a heat source as well as a light source for a general illumination purposes and for an optical equipment and the like has expanded, and incandescent lamps have been used in a heating device for heat treatment such as rapid heating, high temperature holding and rapid cooling or the like. In such an incandescent lamp as above, one in which a double coil is applied as a filament is preferably used.
FIG. 3 illustrates one example of a double coil type incandescent lamp. A filament 20 is formed of a double coil made such that a tungsten wire is applied as a strand, and a primary coil having this wire wound at an equal pitch is further wound into a double form at an equal pitch. The filament 20 is constructed such that a pair of upholding parts 30, 30xe2x80x2 also acting as inner leads are connected to both ends of the filament, and the filament 20 is arranged in a state in which the filament is being hung in a hollow manner within the bulb along the axis of the bulb.
This incandescent lamp 10 was lit and used in a vertical orientation, resulting in that the coil of the filament hung down as the turn-on time expired, and so the incandescent lamp had the disadvantage of the double coil also being extended downwards and being easily deformed.
That is, the incandescent lamp as described above is made such that the filament is oriented in a vertical direction, i.e. an axis of the double coil is oriented in a vertical direction and the net-weight of the coil can be set equated with stress, so that the incandescent lamp has the feature of the load applied to the coil being lowest at the lower end of the filament and having a maximum at the upper end where it is gradually increased as the load is directed more upwardly compared with the lower end thereof.
Due to this fact, although the stress is scarcely generated at the lower end of the filament and prevents the coil from hanging out, the load is increased in a more upward position from the lower end, resulting in that the coil remarkably hangs down, and the largest extension of the coil is found at the upper end of the filament.
In case of the aforesaid filament in the incandescent lamp, the upper end and the lower end thereof are connected to the upholding parts and fixed there so that the number of turns of the double coil and the length of the filament do not change. Accordingly, due to the aforesaid reasons, when the coil hangs down, the pitch of the double coil is gradually narrowed toward the lower part, finally resulting in that the pitch becomes zero and the adjoining upper primary coil and lower primary coil of the double coil get in contact with each other and a short-circuit state or melt-cut state of the filament and the like occur.
In particular, in case the incandescent lamp was used under such a high temperature as one in which the temperature of the filament was 2600xc2x0 C., for example, and its turning-on or turning-off was repeated within a short period of time to light it in a pulse-like manner, a hang-down of the coil occurred remarkably, resulting in the problem of a short lifetime of the lamp.
In view of the aforesaid circumstances, the upholding parts were fixed to not only the upper end and the lower end of the filament but also over its longitudinal direction, and it was possible to provide means for preventing a hang-down of the coil. However, in accordance with this method, the structure of the filament portion becomes complicated and its productivity becomes inferior.
Further, there is the circumstance that if the incandescent lamp is utilized as a heat source, in particular, when the filament length is approximately the same as the diameter of the double coil, the arrangement of many upholding parts at the filament light emitting section is not preferable due to the fact that the thermal efficiency of the lamp is reduced by this arrangement.
In view of the foregoing, it is an object of the present invention to prevent the lifetime of an incandescent lamp from being reduced due to the coil hanging down while using a simple construction and to provide an incandescent lamp having a long lifetime in use.
This invention provides an incandescent lamp in which a filament is stored in a bulb and the filament is held by upholding parts while being oriented in a vertical direction, characterized in that the filament is comprised of a double coil in which a primary coil having a strand wound therein is wound in a double form and the primary coil is more closely wound at the upper part of the filament as compared with that of the lower part of the filament.
With such an arrangement as above, since the shearing stress applied to the coil is reduced when a load is applied to the double coil, resistance against a hang-down characteristic of the coil at the upper part of the filament is increased, the double coil hardly hangs out even if the load applied to it is high, resulting in that a deformation of the filament can be prevented and it becomes possible to prevent the coils from getting in contact with each other. Further, in accordance with the present invention, it becomes possible to restrict the hang-down of the coil and to obtain a long lifetime of the lamp by a simple construction without changing the other configuration of the lamp except for the filament.